1. Field of the Invention
The present invention relates generally to the injection unit of an injection molding machine and, more particularly, to a drive apparatus in the injection unit that accomplishes both the injection and extruder functions.
2. Description of the Related Art
The injection unit of an injection molding machine provides essentially two functions during the course of a normal cycle of operation; namely, injection and extruder. The injection function occurs when the feed screw is moved forward linearly (without rotation) to force plastic melt into the mold. The extruder function is accomplished when the feed screw is rotated to plasticize additional material for the next shot. As the feed screw is rotated during the extruder function, the plastic melt is gradually forced past the end of the screw, creating a pressure or force to move the screw rearward to its pre-injection position as the melt accumulates.
Both the injection and extruder functions require an associated drive apparatus in the injection unit. In hydraulic machines, the movement for the injection function is typically performed by a hydraulic cylinder, while the rotation of the feed screw for extruder run is normally accomplished by a hydraulic motor. More recently, electric motors combined with mechanical systems have been used as the direct power source in the injection unit. Some prior art electric systems have used separate motors for each function; i.e., one motor for rotating the feed screw and a second motor in combination with a mechanism, such as a ball screw, to convert rotary motion into the linear movement required for injection. Other electric machines use a single motor with a massive, complex system of brakes, clutches, and driven elements to accomplish the injection and extruder functions alternately.
While the two motor approach allows fully independent control, the "single" motor system can still accomplish the necessary functions while also providing the potential for a more economical system. However, prior art systems using the single motor concept have required power transmission systems that are mechanically complicated, often requiring duplicate components that eliminate the cost saving potential. Furthermore, such systems have not been able to restrict rotation of the feed screw during injection without adding more costly components. Obviously, the complexity of such systems negatively affects the reliability of the injection unit. Since injection molding machines normally operate in a continuous (automatic) mode with the injection and extruder functions often occurring every few seconds, durability and reliability of the drive apparatus is a key concern.
Accordingly, it is an object of the present invention to provide a drive apparatus that is simple in construction and effectively provides the extruder and injection functions by a single motor.